diff --git a/src/spellfile.c b/src/spellfile.c
index 710e438..bf51f5f 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -4305,7 +4305,7 @@
 	    bl = NULL;
 	else
 	    // Allocate a block of memory. It is not freed until much later.
-	    bl = alloc_clear(sizeof(sblock_T) + SBLOCKSIZE);
+	    bl = alloc_clear(offsetof(sblock_T, sb_data) + SBLOCKSIZE + 1);
 	if (bl == NULL)
 	{
 	    if (!spin->si_did_emsg)